Alumina recovery from oil shale residue

ABSTRACT

An essentially silica-free alumina product is obtained from dawsonite-bearing oil shale by a process which involves rapidly wetting and leaching the retorted residue at about 5*C to about 40*C, with a dilute alkaline leach liquor followed by precipitation of the extracted alumina directly from the leach liquor at about 60*C to about 75*C in the presence of alumina seed.

- United States Patent 1191 1111 3,859,413 Haas *Jan. 7, 1975 [5 ALUMINA RECOVERY FROM OIL SHALE 2,141,132 12/1938 Folger 423/127 RESIDUE 3,201,199 8/1965 Lindsay 423/127 3,545,923 12/1970 Mercier 423/127 Inventor: Frank Haas, A Colo- 3,721,730 3/1973 Haas 423/119 [73] Assignee: The Oil Shale Corporation, New

York, NY. Primary Examiner-Oscar R. Vertiz v Assistant Examiner-Brian E. l-learn Nome ggsf gggggf g g g g Attorney, Agent, or FirmBrumbaugh, Graves,

has been disclaimed. Donohue & Raymond 22 Filed: Au 9, 1972 l 1 g 57 ABSTRACT [21] Appl. No.: 279,194

An essentially silica-free alumina product is obtained from dawsonite-bearing oil shale by a process which [52] U.S. Cl 423/ 122, 423/1 19, 423/629 i l rapidly wetting and leaching the r m t d i- [51] Int. Cl. C01f 7/34, COlf 7/14 due at about 5 to about 40 h a dilute alkaline [58] Fleld of Search 123/122 127; 23/301 R leach liquor followed by precipitation of the extracted alumina directly from the leach liquor at about 60C [56] References cued to about 75C in the presence of alumina seed.

UNITED STATES PATENTS 318,105 2/1927 4 Clalms, N0 Drawings Pedersen 423/127 ALUMlNA RECOVERY FROM OlL SHALE RESIDUE The present invention is directed to a process for recovering high purity, low silicaalumina from the retorted residue-of dawsonite-bearing oil shale.

Dawsonite is a basic sodium aluminum carbonate mineral having the formula NaAlCO (OI-I) Until recently, this sourceof aluminum has been primarily of academic interest because economically attractive quantities have not been available. However, within the past few years, this mineral has been discovered in great abundance in association with the oil shales and nahcolite of the Piceance Creek Basin in northwestern Colorado. In this association, it is considered a low grade alumina ore which is mined with the oil shale. While the extent of this mineral is'not entirely known, data are available to indicate that the aluminum values from this mineral far exceed the bauxite reserves of the United States. At the present time, almost 90% of this countrys bauxite is imported. Development of this reserve would provide a substantial domestic source of aluminum for many years intothe future.

In my co-pending US. Pat. application Ser. No. 102,930, filed Dec. 30, 1970, alumina is recovered from retorted oil shale residue (spent shale) by a process which involves leaching the residue with a soda ash-caustic soda leach liquor to effect the extraction. The alumina is thereafter recovered by precipitation with carbon dioxide. Silica levels in the product are controlled by desilication via seeding prior to the alumina precipitation. The leach operation is conducted for about 1 to 60 minutesat a temperature from about 20 C to about 100 C. While this process is effective in leaching the aluminum values from the retorted residue, it is economicallydesirable to simplify the process by eliminating the carbon dioxide precipitation and desilication procedures without sacrificing product purity in terms of maintaining low silica levels.

It is, therefore, the object of the present invention to provide an economical'process for the recovery of essentially silica-free alumina from dawsonite-bearing oil shale.

In order for raw alumina to be of significant value, it must be of electrolytic cell grade quality. The alumina obtained from the alkaline leaching of dawsonitebearing oil shale will generally meet all specifications with the exception of silica. The silica specification for electrolytic cell grade alumina is about 0.03 weight percent or less silica measured as SiO It has now been found that essentially all of the dawsonitic alumina can be recovered from the solid residue product of oil shale pyrolysis without solubilizing significant amounts of silica. The process involves leaching the retorted residue (spent shale) in an aqueous slurry with a dilute alkaline leach liquor at a temperature of from about 5 C to about 40 C for about 1 minute or less after the residue has been water wetted to solubilize the aluminum values, heating the aluminacontaining liquor to a temperature from about 60 C to about 75 C for about 0.5 to about 4 hours in the presence of alumina seed, i.e., alumina trihydrate, to precipitate the alumina directly from the leach liquor. At pyrolysis temperatures above 350 C, dawsonite is converted to sodium aluminate, alumina and sodium carbonate. The degree of dawsonite conversion to each of these constituents is a function of retorting temperature. As the retorting temperature is increased, the

conversion reaction favors increased formation of sodium aluminate and decreased formation of sodium carbonate and alumina.

The alkaline leach liquor may be initially constituted from water or through the addition of caustic soda. If it is desired to reduce chemical costs, caustic soda may be obtained from the residue by conducting the initial leach with water and thereafter precipitating the alumina from the aqueous leach liquor. Upon precipitation of the alumina, caustic soda is formed which is then recycled to the leach circuit. By repeating this operation, the caustic soda content in the akaline leach circuit may be increased to any desired level and, preferably, to between about 5 to about 25 grams per liter. However, if the foregoing water leach procedure is used, several recycle operations will be required to bring the caustic soda concentration to the desired level in the leach circuit. Alternatively, the leach liquor may be initially consitituted by adding from about 5 to about 25 grams per liter caustic soda to the initial aqueous slurry. The caustic soda addition solubilizes the sodium aluminate, alumina and sodium carbonate. In a continuous process, the alkalinity of the leach circuit can be controlled by bleeding off a portion of the liquor recycled from the precipitation circuit. It has been found desirable to maintain the leach alkalinity below about 25 grams per liter caustic soda to obtain optimum alumina and minimum silica dissolution. When the alkalinity of the recycled leach liquor increases above about 25 grams per liter caustic soda, substantial quantities of silica are solubilized which contaminate the product during precipitation of the alumina. Thus, either after several recycle operations or after the initial addition of caustic soda upon start up of the leach and precipitation circuits, the process is chemically self-sufficient.

The sodium carbonate forms a portion of the leach liquor, although the extent to which it contributes to the extraction procedure is unknown; and depending on the composition of the ore, the sodium carbonate level will usually vary from a low of about 5 grams per liter up to about grams per liter. This level may go higher if substantial quantities of a locally associated mineral, namely, nahcolite (Nal-lCO is present, although it is desirable to restrict the sodium carbonate level of the leach liquor to substantially less than about 150 grams per liter to avoid high alkalinity which adversely affects silica solubility.

The leach operation may be initiated once the retorted residue (spent shale) has been wetted or initiated simultaneously with the wetting procedure. Usually, complete retorting of oil shale is accomplished in about 5 minutes at about 500 C and thereafter cooled prior to disposal or further treatment to extract other minerals. The effectiveness of the process utilized to retort the shale determines to a great extent the difficulty of wetting the residue. It is desirable to have virtually no benzene-soluble oil and less than 5 weight percent fixed organic carbon remaining on the residue. Wetting also depends on the equipment chosen to admix the residue and water or leach liquor. For example, in a continuous circuit the wetting and leaching may be carried out simultaneously by separately feeding the leach liquor and pyrolysis residue to a pump suction; allowing the pump to rapidly mix, wet and leach; filtering the leached residue; and discharging the liquor into the precipitation vessel. When the wetting and leaching operations are conducted separately. the residue is considered wetted as'soon as a dry, dusty appearance is no longer visible.

Once the spent shale has been wetted, the temperature of the leach circuit and time required to effect the leaching or solubilization of the aluminum values become controlling to the process. The leach operation is conducted at a temperature that is considered cold relative to the subsequent precipitation circuit temperature, and as rapidly as physically feasible. It has been found that increasing the leach temperature to above about 40 C increases the soluble silica in the leach liquor, while decreasing the leach temperature to below about C does not significantly enhance the alumina weight percent of the aluminum values originally contained in the retort residue. The alumina seed utilized herein is preferably alumina trihydrate in a seed ratio of from about 1:1 to about 8:1 or higher. Both fine and coarse grade seed may be used, although precipitation is slower using the coarse seed. It has generally been found that the time required for precipitation of the alumina is inversely proportional to the seed ratio, that is, a seed ratio of 1:1 will require about 3 to 4 hours precipitation time to produce a satisfactory product, whereas a seed ratio of 8:1 requires only about 1% hour of precipitation time. i

The process of the invention will be illustrated further by the followingdata and examples.

product, i.e., the alumina to silica weight ratio of the product. It is preferred to carry out theleaching opera- EXAMPLE 1 eh i clsmce lowehtemperfltures i This example illustrates the effect of varying the i g g h a g leach temperature and time upon the amount of aluaso oun t an e a ummum Va ues are eac mina and silicaleached from alumina-containing realmost mstantaneously and that Prolonged a h torted oil shale residue. A high alumina to silica weight P adversely affect pmfluctPumy by solublhzmg ratio is indicative of high alumina and low silica extrac- 1ca. The desired leach time 1s less than I mmute and tion a usually in the Order of a few Seconds after the spent A sample of dawsonite-bearing oil shale was retorted Shale has b wetted e unusually P prodflcts 5 at 480 C for about 5 minutes to provide oil vapors and Produced accPrdanqe the exemples lescnbed a finely divided residue product containing aluminum e e were ebtamed w1th eommerelally avallable values. The residue product was essentially free of bentr1t1on scrubber WlllCh slmultaneously effects very zene soluble hydrocarbons and contained about 4 rapid mixing e leaehihgweight percent fixed organic carbon.

selublhzed alumlhum Values e f e v Portions of the sample were wetted and leached with reetly from the le hq e preelpltahoh the an aqueous alkaline leach liquor containing caustic pl'e of alulhlhaltrlhydrate Seed- The p o W soda. The initial caustic soda concentrations were from c1p1tates asalumma trlhydrate which is thereafter cal- 10 to 20 grams per li Th I h liquor was used to cined to alumina. Since the leaching operation is confir t wet h id product d h to l h thg l duoted at from about C to about the alumimina. Both operations were conducted in a commernum-containing liquor iS W t d n to a separate Vessel cially available attrition scrubber. Visual inspection of and ea to between about C and about C- the wetting procedure indicated that the residue was The precipitation is conducted over a period of from usually completely wetted within about 15 to 30 secabout 30 minutes to about 4 hours depending on the onds. Leaching begins as the particles are wetted. Conweight ratio of alumina trihydrate seeds to alumina in sequently, the length of the leach time is difficult to desolution (hereinafter called seed ratio). The alumina termine with certaintysince some leaching occurs beproduct which precipitates under the foregoing condi-' fore the sample is entirely wetted. For this reason, it tions conforms to the silica specification set for the prowill be noted in the following Table 1 that maximum duction of electrolytic cell grade alumina, i.e., 0.03 leach times are presented. In two instances, times of weight percent SiO or less. less than about 0.1 minute were recorded on samples The optimum alumina precipitation conditions for obtained immediately after the solids appeared to be the purpose of this invention are about C for 1 to completely wetted, i.e., the dusty-like appearance of 2 hours at a seed ratio of 5:1, although temperatures as the solids was rendered wet. The sample portions were low as about 60 C and as high as about C may be leached at 20% to 40% solids concentration at leach used to produce a product having a silica content very 50 temperatures which were varied from 5 C to 60 C. close to 0.03 weight percent. Temperatures below and The percent alumina (A1 0 extraction and the A1 0 above the foregoing specified range results in products and SiO content of the pregnant leach liquor were decontaining more than specified desired silica content, termined as a function of leach temperature and time. as well as poor alumina precipitation. Overall alumina The results of 33 such tests are set forth in the following recovery by this process is in the order of to 55 Table l:

TABLE 1 Max. Leach Leach A1203]- Test Temp. Time Leach Liquor Pregnant Liquor, g/l S102 A; No. "C Min. Solids g/l NaOH A: S10, Ratio Extn 13-51 60 '10 40 20 18.05 0.143 126 95 H09 60 10 20 10 8.10 0.062 131 100 G-229 60 1 40 20 18.04 0.090 200 96 G-228 60 1, 40 10 15.83 0.041 386 88 G-230 40 1 40 20 19.11 0.057 335 99 0226 25 1 40 20 15.85 0.030 528 91 G-221 25 1 40 20 16.48 0.023 717 99 H-4l4 25 1 20 10 8.38 0.015 516 99 1-1-412 5 1- 20 20 7.13 0.012 600 100 71-235 25 0.5 40 20 19.08 0.030 636 92 G-248 25 0.5 40 15 17.76 0.019 935 96 TABLE 1 Continued Max. Leach Leach A1=O=l Test Temp. Time Leach Liquor Pregnant Liquor, g/l S10 A1 No. C Min. Solids g/l NaOH A1 0 SiO, Ratio Ext'n Average of 19 Tests From the above data in Table 1, it is readily apparent that the highest A1 O /SiO weight ratios result from conducting the leaching operation at temperatures from 5 C to 25 C for 1 minute or less after the residue has been wetted.

EXAMPLE 2 This example illustrates the effect of raising the precipitation temperature from that used in the leach opleached with an aqueous alkaline liquor containing 20 grams per liter caustic soda at a solids concentration. The total time required for the wetting and subsequent leaching operations was 1 minute at C. lmmediately after conducting the leaching operation, the pregnant liquor was withdrawn from the leach vessel and heated to varying higher temperatures and the alumina allowed to precipitate directly from the heated liquor in the presence of alumina trihydrate seeds. All precipitations were done with a 5:1 seed ratio. The percent of silica in the alumina product (calculated) as a function of the precipitation 2: and time is reported in the following Table 2:

Table 2 Alumina and Silica Precipitation versus Time and Temperature Precip. Precip. Solution Analyses A1 0 S10 SiO in Temp.,C Time, hrs g M 0 11 g 510 /1 Precip. Precip. Prod. (ca1c.)

F rom the data in the above Table 2, it is readily apparent that the optimum conditions for alumina precipitation and meeting the silica specifications of about 0.03 weight percent are 65 C for l to 2 hours. The data also show that the temperature could go as low as 60 C and as high as 75 C and still produce a satisfactory product, but the silica content would be very close to specification. Below and above this temperature range, silica is above specification and the precentage of alumina precipitated is lower.

EXAMPLE 3 This example illustrates the effect of precipitation time and temperature on alumina recovery and silica contamination of the alumina product using a semicontinuous (batch) leach circuit in which recycled barren-precipitation liqour is cooled and utilized as the leach liquor. Leach temperatures of 25 C and 40 C, respectively, were tested.

Two tests were conducted in which retorted aluminacontaining oil shale residue was leached with recycled liquor at 25 C and 40 C, respectively, at 40% solids concentration for 1 minute and grams per liter of caustic soda (initial concentration). No additional caustic soda was added to the recycle liquor. The pregnant leach liquor was withdrawn to a separate vessel, heated to 70 C to 80 C and then seeded with alumina trihydrate in a seed ratio of 5:1 to precipitate alumina directly from the leach liquor. The barren precipitation liquor was then recycled to additional residue for extracting additional aluminum values. Table 3 presents the results of seven cycles, conducted at a leach temperature of C, whereas Table 4 presents similar data at C.

Table 3 Direct Precipitation at 70C to 80C Recycle Leach Liquor Cooled to 25C Cycle and Precip. Solution Analyses A1 0 A1 0, SiO, in Liquor Type Time, hrs g A1 0 g SiO=/l Extr'n Precip. Prod. (Calc.)

First Cycle Pregnant 20.76 0.045 99 Barren 4.0 8.95 0.044 57 Barren 19.5 7.95 0.044 62 0.008

Second Cycle Pregnant 24.75 0.041 93 Barren 4.0 9.79 0.039 0.013

Third Cycle Pregnant 26.98 0.043 98 Barren 0.5 11.67 0.041 57 0.013 Barren 1.0 11.67 0.041 57 Barren 2.0 11.51 0.041 57 Barren 3.0 11.11 0.039 59 0.025

Fourth Cycle Pregnant 26.05 0.043 95 Barren 0.5 10.33 0.038 60 0.032 Barren 1.0 9.37 0.038 64 Fifth Cycle Pregnant 23.70 0.034 86 Barren 0.5 10.08 0.029 54 0037 Sixth Cycle Pregnant 25.91 0.030 102 Barren 0.6 9.58 0.026 63 0.024

Seventh Cycle Pregnant 23.27 0.032 92 Barren 0.5 9.59 0.028 59 0.029

Table 4 Direct Precipitation at C to C Recycle Leach Liquor Cooled to 40C Cycle and Precip. Solution Analyses A1 0: A1 0 SiO in Liquor Type Time, hrs g A1 0, g SiO /I Extrn Precip. Prod. (Calc.)

First Cycle Pregnant 22.03 0.074 99 Barren 0.5 10.89 0.061 51 0.12

Second Cycle 1 1 Pregnant 29.19' 0.063 91 Barren 0.5 12.88 0.058 56 0.031 Barren 1.0 12.38 0.058 58 0.030

Third Cycle Pregnant 30.27 0.117 91 Barren 0.5 13.64 0.075 55 0.25

Direct Precipitation at 70C to 80C Recycle Leach Liquor Cooled to 40C Cycle and Precip. Solution Analyses A1 0; '71 A1 0: SiO in Liquor Type Time, hrs g Al,O /l g SiO /l Extrn Precip. Prod. (Cale) Fourth C cle Pregnant 35.40 0.100 99 Barren 1.0 17.10 0.093 51 0.038 Barren 2.0 16.70 0.095 53 0.027 Barren 3.0 15.00 0.097 58 0.015 Barren 4.0 13.91 0.093 61 0.033

Fifth Cycle Pregnant 32.44 0.112 96 Barren 1.0 16.53 0.103 49 0.057 Barren 2.0 15.00 0.103 54 0.052 Barren 3.0 15.00 0.098 54 0.080 Barren 4.0 14.02 0.106 57 0033 Sixth Cycle Pregnant 21.73 0.061 97 V Barren 0.5 11.56 0.045 47 0.16 Barren 1.0 10.90 0.045 50 0.15 Barren 2.0 10.87 0.046 50 0.14 Barren 3.0 10.10 0.046 54 0.13 Barren 4.0 9.45 0.050 57 0.09

Seventh Cycle Pregnant 29.95 0.058 94 Barren 2.0 13.24 0.050 56 0.048 Barren 3.0 12.96 0.050 57 0.047 Barren 4.5 11.15 0.050 63 0.043

ature of 40 C, (Table 4), the silica content of the product became quite erratic and did not always meet the desired maximum level of 0.03 weight percent, whereas excellent results were obtained when the recycle liquor 40 was cooled to C (Table 3) prior to each leach operation; It is also shown that extending the precipitation time beyond minutes, i.e., for l to 4 hours, does not significantly increase the amount of alumina precipitated.

This example illustrates the adaptability of the process to dawsonitic oil shales which also contain about 4 weight percent nahcolite (NaHCO In order to successfully extract alumina from this type of ore, a bleed stream from the barren precipitation liquor was taken on each cycle to maintain a low level of alkalinity in the leach liquor, i.e., about 20 grams per liter caustic soda.

A batch leaching test was performed on the retorted residue of dawsonite-bearing oil shale which also contained 4 weight percent nahcolite. The leaching operation was conducted at 25 C and 40% solids concentration for one minute with a leach liquor initially containing 20 grams per liter caustic soda. Precipitation was conducted in a separate vessel for 1% hour with a seed ratio of 5:1. Precipitation temperature during the first five cycles was maintained at C and thereafter at 60 C. A 10% bleed stream was taken from the recycle liquor. The percent alumina extraction and precipitation, and silica in the product through 14 cycles is pres- EXAMPLE 4 ented in the following Table 5:

Table 5 Direct Precipitation Recycle Leach (25C) of Residue Containing 4% Nal-lCO with a 10% Bleed Cycle and Solution Analyses A1 0 A1 0; SiO in Liquor Type g Al Q /l g SiO /l Extrn Precip. Prod. (Cale) First Cycle Pregnant 21.35 0.069 97 Barren 16.58 0.066 22 0.063

Second Cycle Pregnant 32.37 0.066 94 Barren 14.68 0.057 55 0.051

Third Cycle Pregnant 33.53 0.066 95 Barren 14.16 0.066 58 0 Fourth Cycle Pregnant 34.23 0.071 Barren 15.27 0.064 55 0.036

Table 5 Continued Direct Precipitation Recycle Leach (25C) of Cycle and Solution Analyses A1 A1 0 %SiO in Liquor Type g Al O /1 g SiO /1 Extr'n Precip. Prod. (Calc.)

Fifth Cycle Pregnant 33.18 0.082 89 Barren 16.03 0.069 52 0.076

Sixth'Cycle Pregnant 27.22 0.065 80 Barren 12.69 0.060 53 0.034

Seventh Cycle Pregnant 28.13 0.064 84 Barren 13.35 0.064 S 3 0 Eighth Cycle Pregnant 31.15 0.058 94 Barren 14.12 0.055 55 0.018

Ninth Cycle Pregnant 3 l .25 0.061 88 Barren 13.86 0.061 56 0 Tenth Cycle Pregnant 31.35 0.064 91 Barren 14.44 0.062 54 0.012

Eleventh Cycle I Pregnant 30.90 0.065 91 Barren 13.07 0.062 1 58 0.017

Twelfth Cycle Pregnant 30.78 0.060 97 Barren 13.69 0.056 56 0.024

Thirteenth Cycle Pregnant 28.79 0.060 95 Barren 11.86 0.060 59 0 Fourteenth Cycle Pregnant 28.64 0.056 91 Barren 13.66 0.053 52 EXAMPLE This example illustrates the effect of varying the seed ratio on the alumina precipitation time and the silica content of the product.

One test was conducted in which retorted aluminacontaining oil shale residue was leached for 1 minute at C with 20 grams per liter of caustic soda (initial concentration) at solids concentration. Four samples of the pregnant leach liquor were withdrawn to separate precipitation vessels and heated to and maintained at C in the presence of alumina trihydrate seed. Seed ratios of 1:1, 3:1, 5:1, and 8:1 were tested. Reagent grade fine alumina trihydrate seed was used in each test. The results for precipitation times up to 3 hours are presented in the following Table 6:

Table 6 Aluminum and Silica Precipitation versus Seed Ratio and Time Seed Precip. Solution Analyses A1 0 SiO $10 in Ratio Time, hrs g A1 0 g SiO /1 Precip. Precip. Prod. (Cale) From the above data in Table b, it is apparent that a virtually silica-free, high yield alumina product is obtained with a high 8:1 seed ratio and a precipitation of 30 minutes. Satsifactory product yields and purities are obtainable at seed ratios as low as 3:1 for two to three hours precipitation time.

Various modifications and changes in the process of the invention in addition to those pointed out above can be readily made by those skilled in the art without departing from the essence of the invention and accordingly the invention is to be limited only within the scope of the appended claims.

What is claimed is:

l. A process for recovering high purity, low silica alumina from the pyrolysis residue of dawsonitebearing oil shale which comprises:

a. wetting and leaching the pyrolysis residue with a dilute alkaline leach liquor containing from about 5 to about 25 grams per liter of caustic soda at a leaching temperature of from about 5 C. to about 40 C. for a short leaching time period of about 1 minute or less to solubilize the aluminum values of thepyrolysis residue in the leach liquor; and

b. heating the leach liquor containing the solubilized aluminum values to a temperature of from about 60 C. to about C. in the presence of alumina trihydrate seed to precipitate alumina trihydrate directly from the leach liquor.

2. The process as defined in claim 1 wherein the temperature of the leach liquor is maintained at about 25 C.

3. The process as defined in claim 1 wherein the precipitation step (b) is carried out over a period of from about 30 minutes to about 4 hours.

4. The process as defined in claim 1 wherein the ratio of alumina trihydrate seed to alumina in solution varies from about 1:1 to about 8:1.

UNITED STATES PATENT OFFICE 5 9 CERTIFICATE OF CORRECTION Patent No. 3,859,413 Dated January 7, 1975 Inventor) Frank C. Haas It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 6, line 13, solids" should read 40% solids line 23, "precipitation 2:" should read precipitation temperature Col. 8, line 6, "and 20 grams" should read with 20 grams Col. 13, line 1, "Table b" should read Table 6 Signed and sealed this lst day of April 1975.

.,I L i itt'st:

C. I 2:31:55.1. DANIZ RUTI-i C. Ii -SGT? Cotmissioner of Patents Lttnsting C'Tficer and Trademarks 

1. A PROCESS FOR RECOVERING HIGH PURITY, LOW SILICA ALUMINA FROM THE PYROLYSIS RESIDUE OF DAWSONITEBEARING OIL SHALE WHICH COMPRISES:
 2. The process as defined in claim 1 wherein the temperature of the leach liquor is maintained at about 25* C.
 3. The process as defined in claim 1 wherein the precipitation step (b) is carried out over a period of from about 30 minutes to about 4 hours.
 4. The process as defined in claim 1 wherein the ratio of alumina trihydrate seed to alumina in solution varies from about 1:1 to about 8:1. 